The brake vacuum is taken from throttle 1 & 4 via a side port. They are summed by a T-joint before connected to the brake servo. Since throttle 4 is far from brake servo hence it is connected to another pipe then to the T-joint (not shown).

In order to get very stabilized "balanced" vacuum, drill extra hole pointed by the circle below for optimum performance.

Individual throttle body vacuum matching / balancing via mechanical fine tuning through valve opening. The valve limits the amount of air that by-pass the butterfly.
There are two holes behind the butterfly.

Notice the old 4 throttles (shown below) does not have the opening (shown above) and that the injectors output spray is significantly hindered when comparing 4 throttles from generation year 91 against year 93 June till 20vMAP edition. The year 93 unit is so much better.
But this does not mean the old 4 throttle does not have all 4 ISCV holes. They are hidden from current view, but shown by the 4 purple arrows.